I. Field of the Invention
The present invention relates to the field of restorative dentistry and, more particularly, to a class of visible light photocurable organosilicon composite materials suitable for both anterior and posterior tooth restoration.
II. Related Art
Composite dental restorative materials have been the subject of much research in recent years. Early work in the field includes that done by Bowen, exemplified by U.S. Pat. 3,066,112, which describes early composite dental restorative materials. Some of these materials have been gradually accepted as restorative materials for anterior teeth, however, when used as restorative materials for posterior teeth, the materials suffer from a number of shortcomings and, therefore, have not yet met with wide acceptance clinically.
Current composite dental restorative materials generally consist of a monomer, a ceramic filler, a photoinitiator, an activator, and a coupling agent. Upon exposure to light, the photoinitiator and the activator generate the free radicals which initiate the polymerization of the monomer. The polymer acts as the binder for the filler, and the coupling agent is used to bond the polymer and the filler. The monomers are generally high molecular weight dimethacrylates which are polymerizable by heat, chemicals, or light. A commonly used visible light initiator is camphorquinone which is used to initiate the polymerization in combination with an activator such as ethyl N,N-dimethyl aminobenzoate.
Major deficiencies of using composite materials as posterior restorative materials are lack of durability and color stability in the oral environment. Lack of durability is mainly due to loss of substance through wear, and loss of anatomic form due to microleakage caused by polymerization shrinkage and lack of adhesion of composites to tooth structure. Color instability of composite resins has been attributed primarily to oxidation of the residual photoinitiator, and the amine activator.
Significant research effort has been devoted to address the microleakage (marginal leakage) problem caused by polymerization shrinkage and lack of adhesion. Polymerization shrinkage can be minimized by using a high molecular weight monomer such as Bis-GMA, I, below, and by increasing the filler content in the composite. Bonding of dental composites to enamel can be improved by applying the acid-etch technique which creates microporosity on the surface of enamel by treating the surface with orthophosphoric acid. The technique provides mechanical interlocking of composites to the enamel surface. Although good adhesion between the composites and dentin is inherently more difficult, considerable progress has been made in recent years in developing coupling agents which can bond the dental composites to dentin surface. Commercial coupling agents based on phosphate derivatives of monomer dimethacrylate or an isocyanate derivative of an urethane dimethacrylate have been reported to have the property of producing good adhesion between the composite and dentin.
Wear mechanisms of dental composite materials in the oral environment appear to be complex. Wear is defined as unwanted removal of solid materials from the surface as a result of mechanical action and can include adhesive, abrasive, fatigue, corrosive, or chemical action. Many wear mechanisms have been proposed and postulated; but it is generally recognized that wear of polymer matrix through chemical and mechanical degradation, and loss of ceramic filler due to debonding between the filler and the polymer, are two important contributing factors. As the polymer matrix is removed from the surface of the restoration by abrasion and chemical degradation, more filler particles are exposed. Although filler particles are quite abrasion resistant, debonding between the polymer and the filler particle results in loss of filler particles thereby exposing more polymer surface. A high resistance to chemical degradation, and strong bonding between the polymer and the filler in the oral environment, are the two most important requirements for high performance dental composites.
Current polymer matrices used for dental restorative materials are based on free radical addition polymerization and cross-linking of high molecular weight monomers bisphenol A Bis(2-hydroxylpropyl) methacrylate, Bis-GMA, (I) or urethane dimethacrylates (II) (UDMA). Each monomer molecule of Bis-GMA and urethane dimethacrylate possesses two methacrylate groups (MA) (III), and each methacrylate group contains a carbon-carbon double bond where polymerization and cross-linking take place. ##STR1##
These high molecular weight monomers are highly viscous liquids, and diluents are often added to the composite for ease of handling and mixing. Diluents currently in use are generally dimethacrylate monomers of lesser viscosity, e.g. triethylene glycol dimethacrylate (TEGD) (IV). EQU MA--CH.sub.2 CH.sub.2 --O--CH.sub.2 CH.sub.2 --O--CH.sub.2 CH.sub.2 --MA (IV)
The viscous nature of the monomers and the rigid structure of the polymer backbones, however, also lead to a rather low degree of conversion associated with the polymerization of the monomer of approximately 50-70%. As the polymerization and cross-linking proceed, the diffusion rate of propagating free radicals, the unreacted monomer molecules, and the pendant methacrylate species are drastically reduced. The glass transition temperature increases with the degree of polymerization, and the segmental mobility of the polymer chains is also retarded. Consequently, the polymer matrix in the resulting composite materials retains a considerable number of unreacted methacrylate groups. The residual carbon-carbon double bonds in the unreacted methacrylate groups are susceptible to chemical degradation thus contributing to wear.
One polysiloxane material in the form of the amounts of the hydrophobic monomer Bis(3-methacryloxypropyl) tetramethyldisiloxane Bis-MPTMS (V) ##STR2## has also been used as a diluent for the base resins Bis-GMA (I), Bis-IGMA (a non-hydroxylated isomeric analog of Bis-GMA), and urethane dimethacrylate (UMDA) (II), in the formulation of certain dental composite restoratives. This was reported by J. S. Kuo, et al., in "Evaluation of Siloxane Containing Dental Composites", Journal of Dental Research Abstracts, 6A, Abstract No. 30 (1985).
The composites of Kuo are decidedly resin based materials which contain only a minor amount of the material Bis-MPTMS (V) which is added to reduce the high viscosity of the resin in the manner of the other diluents enumerated above. While some success may have been achieved cross-linking these materials and including fillers such as silanized radiopaque glass, the restorative materials of these combinations have been found to be generally lacking in the necessary hardness and mechanical properties demanded of permanent dental restorative materials. Also, polymers of the monomer Bis-MPTMS exhibit glass transition temperatures which are lower than desired.
Low degree of conversion has been recognized as a major polymerization shortcoming in prior dental composite resin technology. Low degree of conversion is intimately related to the flexibility of the polymer backbone. The high level of unreacted dimethacrylate groups associated with the low degree of conversion decreases the hardness, resistance to swelling, and increases the rate of wear. Thus, an alternate monomer which could yield high degree of conversion upon polymerization would be highly desirable as the polymer backbone matrix.
The mechanical properties of polymers can be significantly enhanced by the addition of reinforcing agents to form composite materials. In dental composite restorative materials, the addition of ceramic fillers has the effect of reducing polymerization shrinkage, decreasing thermal expansion, and increasing the hardness, strength and wear resistance of the materials. This superior performance, however, depends critically on the bonding between the polymer and the filler. Strong bonding between the polymer and the filler is needed for stress transfer across the interface thereby allowing the filler to share the stress thus providing the reinforcing effect.
The interaction between the polymer and the filler depends largely on the chemical structures of the polymer and the filler. In dental restorative materials, microcrystalline quartz, pyrogenic silicas, and radiopaque barium glasses have been frequently used as the fillers for the dimethacrylate polymers. The bonding between the polymers (derived from Bis-GMA and urethane dimethacrylates) and ceramic fillers, however, is inadequate without a coupling agent. Materials such as organosilanes have long been used as coupling agents in the dental composite materials to enhance the bonding.
Silane coupling agents currently used in bonding an inorganic substrate to a polymer have a general formula R(CH.sub.2).sub.n Si(OR').sub.3, where R and R' are organoalkyl groups. The R groups usually contain carbon-carbon double bonds for bonding the polymer. The R'O groups are usually alkoxy or acetoxy groups which are hydrolyzable by water. The silane compound, .gamma.-methacryloxypropyl trimethoxysilane (MPTS), CH.sub.2 =C(CH.sub.3)COO(CH.sub.2).sub.3 Si(OCH.sub.3).sub.3, is a coupling agent widely used in dental composites. The coupling agent MPTS comprises a methacrylate group through which the coupling agent is bonded to the methacrylate group of the monomer BIS-GMA (I) or urethane dimethacrylate (II). The coupling agent also contains three methoxy groups which, upon hydrolysis, generate silane triols. The silanol groups can undergo condensation reactions with silanol groups from the surface of the ceramic substrate such as silicon dioxide. The siloxane linkages formed by the condensation reactions provide the bonding between the coupling agent and the filler.
Debonding can occur either at the polymer-silane interface, or at the filler-silane interface, or both. However, it is generally agreed that debonding at the filler-silane interface due to hydrolysis of siloxane linkages in the oral environment is more probable. Hydrolytic stability of the siloxane bonds, therefore, is an important factor in determining material wear.
Hydrolytic stability of the siloxane bonds depends largely on the chemical nature of the filler and the silane coupling agent; however, it is known that the stability can be maintained by minimizing the exposure of the siloxane bonds to water. Thus, a hydrophobic polymer which can repel water, and thus minimize the hydrolysis of the siloxane bonds would be desirable as the polymer matrix.
Whereas silane compounds have found use as adhesives, and are suitable for use as coupling agents with respect to coupling ceramic filler materials to cross-linked siloxane materials, they themselves lack the properties necessary for successful use as primary restorative materials.